A conventional device for generating the second harmonic of a semiconductor laser radiation is described in Japanese Patent Laid-Open No. 142,284/1976. This known device is shown in FIG. 10 and is a hybrid integrated device consisting of a semiconductor laser 1001 of AlGaAs and a device 1002 for second-harmonic generation. The device 1002 is a thin-film optical waveguide made from LiNbO.sub.3 or the like. The laser radiation of wavelength 870 nm emitted by the semiconductor laser 1001 enters the device 1002 for second-harmonic generation. Then, the radiation is converted into a wavelength half the wavelength of the laser radiation, i.e., 435 nm, by the nonlinear optical effect of the thin film of LiNbO.sub.3 and emitted as an outgoing radiation 1003.
Another known device for generating the second harmonic of a semiconductor laser radiation is described in Japanese Patent Laid-Open No. 112,023/1985 and shown in FIG. 11. This device is a hybrid integrated device consisting of a semiconductor laser 1101 of AlGaAs and a device 1102 for second-harmonic generation. The device 1102 is a thin-film optical waveguide of LiNbO.sub.3 or the like. A condenser lens 1103 is mounted between them to efficiently introduce the light from the laser into the optical waveguide. If the incident spot matches the waveguide, then the optical density inside the waveguide can be increased. This enhances the efficiency at which the laser light is converted into the half wavelength.
These prior art techniques have the following problems. In the device shown in FIG. 10, it is quite difficult to make the lasing active layer flush with the thin-film optical waveguide. In addition, their positional relation is easily affected by variations in the ambient temperature and other variations. If their positional integrity is lost at all, then the laser light is not introduced into the waveguide. Further, the efficiency of coupling to the waveguide is low, because the emitted semiconductor laser radiation is spread out. This also leads to a reduction in the conversion efficiency.
In the device shown in FIG. 11, the optical density inside the optical waveguide of a thin film can be increased by the condenser lens. In this case, it is necessary to focus the laser radiation onto the waveguide having a thickness of the order of 1 .mu.m. Therefore, the alignment is very difficult to perform. Furthermore, the positional integrity is easily destroyed by variations in the ambient temperature.
Accordingly, it is an object of the present invention to provide a monolithic integrated device which consists of a semiconductor laser and a device for second-harmonic generation, and in which the laser radiation is coupled to the device for second-harmonic generation at a high efficiency, the optical density in the optical waveguide of the thin film being high, the integrated device converting the laser radiation into the second harmonic at a high efficiency, the coupling efficiency being unaffected by environmental variations.
It is another object of the invention to provide a method of fabricating the integrated device described just above.